Numerous communication protocol standards have developed to enable using the pre-existing plain old telephone system (POTS) infrastructure for carrying digital data. Although the public switched telephone network (PSTN) is digital in nature, the connection between subscribers and the central office that serves as an entry point to the PSTN is analog. As a result, modems are used for bi-directional communication of digital data on the analog channel between the subscriber and the central office. Modems convert the communicated information between the digital and analog domains in accordance with the particular communication protocol.
Some communication protocols are designed to rely on the voiceband region of the analog channel to convey information. The data transmission rate of such voiceband modems, however, is constrained by the relatively narrow bandwidth (i.e., less than 4 kHz) of the voiceband region.
Digital subscriber lines (xDSL) services provide significantly higher data transmission rates by utilizing communication bandwidth beyond the voiceband. Frequently, xDSL services may simultaneously co-exist with voiceband communications on the same subscriber line that connects the subscriber to the central office.
Modems or other devices designed for communicating digital data on the analog channel utilize an analog front end for transmitting as well as receiving information from the subscriber line. The analog front end conditions signals communicated to or from the subscriber line before providing the conditioned signal to the subscriber line for transmission or to a digital signal processor for interpretation.
Typically, circuitry incorporated into an xDSL modem is specifically tailored to a single xDSL protocol. A modem supporting Asymmetric Digital Subscriber Line (ADSL) for example, would not be compatible with a modem supporting an Integrated Digital Services Network (ISDN) protocol or a High bit rate Digital Subscriber Line (HDSL) protocol.
Support for different protocols can be accomplished by providing distinct modems, each of which supports a single xDSL protocol. Even if the service provider were capable of providing a different xDSL service on the same physical line, a different modem would be required to support the service. The provider must thus maintain an inventory of modems to support varying xDSL standards even though the xDSL protocol is largely determined at the time of installation and is somewhat constrained by the distance of the subscriber from the xDSL provider.